Patent GB-764991-A (GB)
Title: Improved automatic telephone system and electronic selector switches therefor
IPC: H04Q3/52
Filed: 1954-03-18
Abstract: 764,991. Automatic exchange systems. CATRAVAS, S. J. March 18, 1954 [March 23, 1953; May 28, 1953], No. 7975/54. Class 40 (4). [Also in Group XL (a)] In an automatic system of the director type the usual electro-mechanical selector switches with two-lead inputs are replaced by gas-discharge speech tubes, so providing a path for the transmission of intelligence in both directions between an inlet and a selected outlet. Seizing connection to a group selector, Fig. 1. When a subscriber on line Lx closes his loop, A pulls up to operate B which connects H.T. to the gas discharge diodes Vx1-VxN, firing at random any tube, say VxN, associated with an idle connector. All other tubes in the same linefinder and all other tubes associated with the same line LN are disabled. Contact 62 opens to prevent line Yx completing an incoming call. Speech or impulses on the subscriber's line Lx modulates the D.C. through the tube VxN and transformer TN to reproduce the speech current or impulses on the balanced line LN. Relay DN pulls up when tube VxN fires to close contact dn1, so seizing the group selector. Relay A responds to dialled impulses, DN following. Incoming call. If the subscriber's line Lx is free when called, tube current from the final selector flows through the unbalanced line Yx to energize relay X and so apply ringing current from transformer Tr to the line Lx, part of the current being returned as tone to the calling line. When the called subscriber answers A operates to disconnect transformer Tr and relay B pulls up without any effect on the circuit. Group selector, Fig. 2. When the group selector is seized by the line LN, relay E operates to connect H.T. to tube VN which fires on standing cathode kS and operates relay G. G operates F and returns dial tone from transformer Td. E responds to dialled impulses, the transfer electrode bias potential dropping at each break period to cause the glow to move from a cathode to the adjacent transfer electrode and the glow moving to the next cathode at the succeeding make period. At the end of the digit train G falls back slowly and the potential of the glowing cathode rises to firing value for the associated group of diodes, say VO1-VOm. Of these diodes with a free line on their cathodes one at random fires, say VOm, and the increased current operates relay H. Transformer TOm now links the group selector to the balanced line LOm and relay JOm operates to seize the following switch. If all lines are busy H remains unoperated and busy tone is returned from transformer Tb. Final selector, Fig. 3. Current on line LOm operates relay K which fires tube V on its standing cathode. Relays P and Q operate, contacts p3 preventing the next impulse train operating any of tubes V1-V0. This impulse train moves the cathode glow, say, to kO after which P restores slowly and the selected tube V0 fires on its standing cathode to energize relays Sand U. The next impulse train passes through tubes V, V0 in series to step the glow in the latter tube. At the end of the digit the glow rests, say, on cathode kOO and relay S falls back to hold the tubes V, V0 in series through resistors RKOO and R10. If the called line x is free, diode Vx fires to establish contact with the called subscriber, as already described with reference to Fig. 1, and relay W operates. If the called line is busy, W does not operate and busy tone is returned from transformer Tb. Modifications.-The diodes in the group selector may be replaced by triodes (Fig. 4, not shown), the anodes of which are connected together and supplied with H.T. independently of the tube VN. The grids of triodes in the same group are commoned and connected to the appropriate cathode of tube VN. Release of G at the end of the impulse train applies extra H.T. to the anodes and one out of those with a free line in the selected group fires and operates H which extinguishes tube VN. The final selector of Fig. 3 may be modified by the use of triodes (Fig. 5, not shown). Multicathode tubes VG, VP respond in turn to two impulse trains to mark the grids and anodes of appropriate tubes, one tube being thereby selected. If the called line is idle relay W operates to extinguish VG, VP which are thus excluded from the talking circuit.